1. Field of the Invention
The present invention relates to a variable magnification optical system used in a video camera, an electronic still camera or the like, and an imaging apparatus. In particular, the present invention relates to a variable magnification optical system that can be used in a wide wavelength range of from a visible wavelength band through a near-infrared wavelength band, and an imaging apparatus including the variable magnification optical system.
2. Description of the Related Art
Conventionally, surveillance cameras have been used for preventing crimes, recording or the like. An optical system for such a surveillance camera needs to be small and structurable at low cost. Further, the optical system needs to have a large relative aperture to identify a subject even in low illumination photography conditions, and also to have a wide angle of view so that a wide range can be photographed. The optical system also needs to have high optical performance. Further, since a demand for a surveillance camera with a variable magnification function has increased in recent years, a variable magnification optical system is becoming a main trend, and such an optical system also needs to have a high variable magnification ratio.
When an optical system is mounted on a surveillance camera used for both day and night, such a surveillance camera is often set at facilities without any people, and photography with visible light is performed during the daytime and photography with near-infrared light is performed at night. Therefore, the optical system needs to excellently correct chromatic aberrations for a wide wavelength range including a visible wavelength band through a near-infrared wavelength band. Especially, excellent correction of longitudinal chromatic aberrations is demanded.
As a zoom lens for a camera used in the aforementioned fields, for example, a two-group zoom lens consists of a negative first lens group and a positive second lens group is known (please refer to U.S. Patent Application Publication No. 20070047096 (Patent Document 1), Japanese Unexamined Patent Publication No. 2009-271165 (Patent Document 2), and U.S. Patent Application Publication No. 20080212203 (Patent Document 3)). Patent Document 1 discloses a zoom lens in which a first lens group consists of a negative lens, a negative lens, a negative lens, and a cemented lens of a positive lens and a negative lens cemented together, which are in this order from an object side. Patent Document 2 discloses a zoom lens in which a first lens group consists of a negative lens, a negative lens, a negative lens, a positive lens and a negative lens in this order from an object side, and in which a double-concave lens is arranged on the most image side in the first lens group. Patent Document 3 discloses a zoom lens in which a first lens group consists of a negative lens, a negative lens, a negative lens, a positive lens and a positive lens in this order from an object side.